Liquid crystal compounds

ABSTRACT

The use of a compound of formula (I)  
                 
 
     wherein R 1  is an alkyl or alkenyl group, either of which may be optionally interposed with one or more oxygen or sulphur atoms, R 2  is an alkyl or alkenyl group, Y′ and Y″ are independently selected from oxygen or sulphur, n is an integer from 1 to 5, X is a direct bond or a C 1-4 alkylene or C 2-4 alkenylene chain, and A is group comprising one or two rings which are independently selected from aryl, heterocylic or cycloalkyl rings, and when there are two rings, they are bonded together directly or by way of a C 1-4 alkylene or C 2-4 alkenylene group, for increasing the u.v. stability of a liquid crystal mixture.  
     The compounds of formula (I) are u.v. stable and are useful in liquid crystal devices which are exposed to high levels of u.v. light such as phosphor display cells.  
     Certain compounds are novel and these form a further aspect of the invention.

[0001] The present invention relates to liquid crystal mixtures whichare stable to u.v. light, to liquid crystal devices containing these andto methods for increasing the u.v. stability of liquid crystal mixtures.Certain compounds useful in these mixtures are novel and these, togetherwith processes for their preparation, form a further aspect of theinvention.

[0002] The term “liquid crystals” is well known. It refers to compoundswhich, as a result of their structure, will align themselves in asimilar orientation, preferably at working temperatures, for example offrom −40 to 200° C. These materials are useful in various devices, inparticular the liquid crystal display devices or LCDs. Liquid crystalscan exist in various phases. In essence there are three differentclasses of liquid crystalline material, each possessing a characteristicmolecular arrangement. These classes are nematic, chiral nematic(cholesteric) and smectic.

[0003] Broadly speaking, the molecules of nematic compounds will alignthemselves in a particular orientation in a bulk material. Smecticmaterials, in addition to being orientated in a similar way, will alignthemselves closely in layers.

[0004] A wide range of smectic phases exists, for example smectic A andsmectic C. In the former, the molecules are aligned perpendicularly to abase or support, whilst in the latter, molecules may be inclined to thesupport. Some liquid crystal materials possess a number of liquidcrystal phases on varying the temperature. Others have just one phase.For example, a liquid crystal material may show the following phases onbeing cooled from the isotropic phase:—isotropic—nematic—smecticA—smectic C—solid. If a material is described as being smectic A then itmeans that the material possesses a smectic A phase over a usefulworking temperature range.

[0005] Such materials are useful, in particular in display devices wheretheir ability to align themselves and to change their alignment underthe influence of voltage, is used to impact on the path of polarisedlight, thus giving rise to liquid crystal displays. These are widelyused in devices such as watches, calculators, display boards orhoardings, computer screens, in particular laptop computer screens etc.The properties of the compounds which impact on the speed with which thecompounds respond to voltage charges include molecule size, viscosity(Δn), dipole moments (Δε), conductivity etc.

[0006] In some cases however, the devices are subject to high levels ofradiation such as ultra-violet radiation. Many liquid crystal compoundsare not stable under these circumstances and can deteriorate rapidly.

[0007] Some examples of alkylene glycol liquid crystal compounds arefound in EP-A-0385471 and JP-64-22835. EP-A-0824141 describes a complexseries of liquid crystal mixtures, some of which may contain alkyleneglycol compounds.

[0008] The applicants have found that certain of these structural types,as well as having useful liquid crystal properties, also haveparticularly good resistance to u.v. light, and so may be used toenhance the u.v. stability of liquid crystal mixtures.

[0009] The present invention provides the use of a compound of formula(1)

[0010] wherein R¹ is an alkyl or alkenyl group, either of which may beoptionally interposed with one or more oxygen or sulphur atoms, R² is analkyl or alkenyl group, Y′ and Y″ are independently selected from oxygenor sulphur, n is an integer from 1 to 5, X is a direct bond or aC₁₋₄alkylene or C₂₋₄alkenylene chain, and A is group comprising one ortwo rings which are independently selected from aryl, heterocylic orcycloalkyl rings, and when there are two rings, they are bonded togetherdirectly or by way of a C₁₋₄alkylene or C₂₋₄alkenylene group, forincreasing the u.v. stability of a liquid crystal mixture.

[0011] As used herein the term “alkyl” refers to straight or branchedchain alkyl groups, suitably containing up to 20, more suitably up to 10and preferably up to 6 carbon atoms. The term “alkylene” refers to alkylgroups which are divalent and “cycloalkyl” refers to alkyl groups whichhave at least 3 carbon atoms, and which are cyclic in structure. Theterm “alkenyl” refers to straight or branched unsaturated chains havingfrom 2 to 20 and preferably from 2 to 10 carbon atoms. The term “aryl”refers to aromatic rings such as phenyl and naphthyl, but preferablyphenyl.

[0012] References to “heterocyclic groups” refer to rings, whichsuitably contain from 4 to 8 atoms, up to three of which are heteroatomsselected from oxygen, nitrogen or sulphur. They may be saturated orunsaturated, but are preferably saturated.

[0013] Suitably, rings in group A are para substituted, and where theyare saturated, substituents are preferably in a trans relationship toeach other. Particularly preferred examples of group A in formula (I)are groups of formula (i), (ii), (iii), (iv) or (v)

[0014] where X′, X″ and X′″ are each selected from a direct bond, a C₁₋₄alkylene chain or a C₂₋₄alkenylene chain, and each group R_(a), R_(b),R_(c) and R_(d) are independently selected from CH₂ or oxygen. Inparticular where one of R_(a) or R_(b), or R_(c) or R_(d) is oxygen, theother is also oxygen so that the group A is or includes a dioxane ring.

[0015] In particular however, R_(a), R_(b), R_(c) and R_(d), wherepresent, are CH₂ groups.

[0016] Suitably Y′ and Y″ are oxygen.

[0017] In a particularly preferred embodiment, n is 2.

[0018] Suitably R² is a C₁₋₁₀alkyl, preferably C₁₋₆ alkyl and mostpreferably C₁₋₃alkyl. Alternatively, R² is alkyl having at least 5carbon atoms, for example from 5 to 10 carbon atoms.

[0019] Preferably, X is a direct bond or a C₁₋₂alkylene chain, and mostpreferably X is a direct bond.

[0020] Where X is a C₂₋₄alkenylene chain, it is suitably a group ofsub-formula (x), (xi) or (xii)

[0021] The cyclohexyl ring illustrated in FIG. 1 is suitably in thetrans configuration. Thus, in particular, the compound of formula (I) issuitably a compound of formula (IA)

[0022] where R¹, R², A, X, Y′, Y″ and n are as defined above.

[0023] In a preferred embodiment, R¹ is a straight chain C₁₋₆alkyl groupand most preferably a C₃₋₅alkyl group, which optionally contains one ortwo oxygen or sulphur atoms. For example, a particular example of agroup R¹ is a group of formula (II)

[0024] where n′ is an integer from 1 to 5, Y′″ and Y″″ are independentlyselected from oxygen or sulphur and R³ is alkyl, in particularC₁₋₄alkyl. Preferably n′ is 2. Preferably Y′″ and Y″″ are oxygen.

[0025] Other particular examples of groups R¹ are C₃₋₅alkyl.

[0026] Yet further examples are C₈₋₂₀alkyl.

[0027] In an alternative embodiment, R¹ is an alkenyl group and inparticular a C₂₋₁₀ alkenyl group. Suitably the alkenyl group containstwo or more double bonds, and is preferably a diene of formula (VI)

—(CH₂)_(m)—CH═CH—CH═CH—(CH₂)_(p)—CH₃   (VI)

[0028] where m and p are 0 or an integer from 1-5, provided that m+pdoes not exceed 5. Preferably m and p are 0.

[0029] Particularly suitable rings A in formula (I) are groups ofsub-formula (i), (iii) and (v), and preferred rings A are groups ofsub-formula (i). In groups of sub-formula (iii), (iv) and (v), X′, X″and X′″ are suitably direct bonds or C₁₋₂alkylene groups. Particularly,these are direct bonds. Where these are C₂₋₄alkenylene groups, these aresuitably selected from groups (x), (xi) and (xii) as listed above inrelation to X.

[0030] Thus particular examples of compounds of formula (I) arecompounds of formula (III)

[0031] where R¹ and R² are as defined above.

[0032] Particular examples of compounds of formula (III) are set out inTable 1. TABLE 1 Compound No. R¹ R² 1 C₃H₇  CH₃ 2 C₅H₁₁ CH₃ 3 C₃H₇ C₂H₅4 C₅H₁₁ C₂H₅ 5 C₃H₅  C₃H₇

[0033] These compounds are suitably added to liquid crystal mixtures inorder to increase the u.v. stability of those mixtures. Other componentsof the mixture will include liquid crystal compounds of variousstructures as are well known in the art. Suitably the compound offormula (I) is added in sufficient quantities to produce a significanteffect on the u.v. stability of the mixture. Therefore, the mixture willsuitably contain the compound of formula (I) in an amount of from 5-80%,preferably from 5-40%, and most preferably from 10-30% by weight.

[0034] Compounds of formula (I) may be prepared by conventional methods.However, in accordance with an aspect of the invention, they areprepared by reacting a compound of formula (IV)

[0035] where R¹, A, X and Y′ are as defined in relation to formula (I);with a compound of formula (V)

z-(CH₂)_(n)—Y″—R²   (V)

[0036] where Y″ and R² is as defined in relation to formula (I) and Z isa leaving group. The reaction is suitably effected in an organic solventsuch as tetrahydrofuran in the presence of a strong base such as analkali metal hydride for instance, sodium hydride. Suitable leavinggroups Z include halo such as chioro, bromo or iodo, mesylate andtosylate, and in particular are halo groups such as bromo.

[0037] Compounds of formula (IV) are either known compounds or they canbe prepared from known compounds by conventional methods.

[0038] Compounds of formula (I) have liquid crystal properties and inparticular are nematic compounds. Thus they may be included in mixturesused in liquid crystal devices including liquid crystal display cells,such as twisted nematic cells (TN) (such as standard TN cells),supertwist nematic cells (STN) and supertwist birefringence effect (SBE)cells.

[0039] Such devices form a further aspect of the invention.

[0040] The compounds of formula (I) have good stability in the presenceof radiation such as ultraviolet radiation. In particular they do notabsorb radiation at the wavelengths used in many liquid crystal devices.These properties make them particularly useful, either alone or inmixture with other liquid crystal compounds, in liquid crystallinedisplays that are exposed to high levels of ultraviolet radiation, suchas those used in outdoor displays. In addition, they may be utilized inconjunction with phosphorescent substrates or phosphor layer liquidcrystal devices, such as those described in U.S. Pat. No. 4,830,469, WO95/27920, EP-A-1 85495 and European Patent No 0755532.

[0041] These devices are particularly suitable for display cells andespecially colour display cells as they overcome problems associatedwith the use of liquid crystals as shutters for transmitting light to aviewer. The light scattering or birefringent properties of thesecompounds is useful in this respect and may be controlled by applicationof an electrical field. However the liquid crystal material is sensitiveto the angle at which light passes through it, and therefore there maybe difficulties with viewing angle when these are viewed directly.

[0042] These problems are overcome in devices such as those mentionedabove, by directing light from a light source, usually an ultravioletlight source, through the liquid crystal layer onto self-radiatingelements or phosphors. These are activated by light reaching themthrough the liquid crystal layer, and thereafter emit light at adesirable viewing angle. Each phosphor therefore can constitute a pixelin a display or a combination of red, blue and green phosphors may begrouped to form a pixel which can emit light at any colour of thespectrum, depending upon the relative stimulation of each.

[0043] Thus a preferred liquid crystal device according to the inventionis a display cell comprising a layer of a liquid crystal materialwherein the liquid crystal material comprises a compound of formula (1),means for addressing the liquid crystal material so as to allow light topass through it when appropriately addressed, and an emitting layercomprising phosphor elements, arranged to receive light passing throughthe liquid crystal layer.

[0044] Such devices are illustrated schematically in FIG. 1 hereinafter.These devices may be arranged differently depending upon the intendedapplication.

[0045] Suitably in these devices, the liquid crystal material iscontained between two parallel, spatially separated transparentsubstrate plates (1), either in individual cells or in a continuouspanel. Liquid crystal material (2), such as a compound of the presentinvention is provided in the cell, and the orientation is controllableby addressing means such as electrodes arranged on either side of thelayer (not shown). Light from a light source is supplied in thedirection of the arrow, and is either internally reflected by the liquidcrystal material, or diverted to phosphors (3) on an emitting layer (4),depending upon the activation of the liquid crystal material. Thephosphors may then emit light at a preferred viewing angle.

[0046] Devices may also contain polarisers and/or dichroic ultravioletlight absorbers (as described in U.S. Pat. No. 4,830,469).

[0047] A particularly preferred device further comprises means forcollimating activating light towards the phosphors. Various arrangementsfor such collimating means are described in WO 95/27920. They includelenses, which may be arranged in or on one of the layers.

[0048] Light from an ultra-violet light source, is supplied to theliquid crystal layer, either directly onto the back or from the edgeusing for example the transparent backing plate as a light guide. Theaddressing means control the orientation of the liquid crystal material,within each cell or region of the panel as is well understood in theart. As a result, light may or may not be directed onto a particularphosphor element, which is either activated to emit light or remaindark, respectively. By appropriate control of the addressing means, eachpixel point has individual visible light output characteristics at anygiven point in time.

[0049] Such devices may include computer or television screens, andthese may contain hundreds of thousands of individual pixels, whichcontrol the amount of red, green or blue light reaching a very smallarea of the screen, for example of 100 μm or less. In such cases, one ofthe electrodes used to address the liquid crystal material may beconnected together in columns, and the other connected in rows (whererows and columns are perpendicular to each other) in order to reduce thenumber of electrical connections required. However, in order to ensurethat pixels are controlled individually, these need to be multiplexed asunderstood in the art. Multiplexing generally achieved by applying avoltage which cycles between the desired voltage and zero many times persecond. As each row receives the required voltage, a positive ornegative voltage is applied to each column so that individual pixelswithin the row are addressed in the required manner. This means that theliquid crystal of all the “on” pixels will subject to a voltage inexcess of the threshold voltage for that compound. All rows in thedisplay are scanned to refresh the pixels.

[0050] Certain compounds of formula (I) are novel and these form afurther aspect of the invention. In particular, the invention furtherprovides a compound of formula (VII)

[0051] wherein R¹⁰ is an alkyl or alkenyl group, either of which may beoptionally interposed with one or more oxygen or sulphur atoms, R¹¹ isan alkyl or alkenyl group, Y′, Y″, n, X and A are as defined above inrelation to formula (I) provided that when A is cycloalkyl, X is adirect bond or a C₂alkylene chain, n is 2, Y′ and Y″ are both oxygen,R¹¹ is methyl, and R¹⁰ is an alkyl group, it contains more than 3 carbonatoms.

[0052] Particular groups of compounds of formula (VII) are compounds offormula (VIIA)

[0053] wherein R¹² is an alkyl or alkenyl group, either of which may beoptionally interposed with one or more oxygen or sulphur atoms, R¹³ isan alkyl or alkenyl group, Y′, Y″, n, and X, and A is a group comprisingone or two rings which are independently selected from aryl orheterocylic, and when there are two rings, they are bonded togetherdirectly or by way of a C₁₋₄alkylene or C₂₋₄alkenylene group.

[0054] Preferred examples of R¹² and R¹³ are as defined above inrelation to R¹ and R². Similarly preferred examples of X, Y′, Y″ and nare as defined above in relation to corresponding variables in formula(I). Preferred examples of A′ are groups of sub-formula (ii), (iii),(iv) or (v) as defined above. Alternatively, A′ may be a dioxane ring.

[0055] An alternative group of compounds of formula (VII) are compoundsof formula (VIIB)

[0056] where X, Y′, Y″ and n are as defined above in relation to formula(1), R¹⁴ is an alkyl or alkenyl group, either of which may be optionallyinterposed with one or more oxygen or sulphur atoms, and R¹⁵ is an alkylor alkenyl group, provided that when R¹⁵ is methyl and R¹⁴ is alkyl ofat least 4 and preferably at least 8 carbon atoms.

[0057] Suitably R¹⁴ is alkenyl.

[0058] More suitably R¹⁴ is alkyl of at least 4 and preferably at least8 carbon atoms.

[0059] Suitably R¹⁵ is alkenyl

[0060] More suitably R¹⁵ is alkyl of at least 2 and preferably at least5 carbon atoms.

[0061] Preferred variables for X, Y′, Y″ and n are as defined above inrelation to formula (I).

[0062] Compounds of formula (VII), (VIIA) and (VIIB) may be prepared byconventional methods, for example by methods analogous to thosedescribed above in relation to formula (I). These novel compounds haveliquid crystal properties and therefore their use in liquid crystaldevices, either alone or in admixture with other liquid crystalcompounds forms a further aspect of the invention.

[0063] The invention will now be particularly described by way ofexample.

EXAMPLE 1 Preparation of trans,trans4′-(2-methoxyethoxy)-4-pentylbicyclohexane (Compound No. 2 in Table1)

[0064] A solution of trans4-(4-pentylcyclohexyl)-1-cyclohexanol (1.00 g,3.97×10⁻³ mol) in terahydrofuran (10 cm³) was added dropwise to asuspension of sodium hydride (0.14 g, 5.95×10⁻³ mol) in terahydrofuran(15 cm³) at room temperature and under a nitrogen atmosphere. Thesolution was then left to stir (2 hrs) before 1-bromo-2-methoxyethane(0.83 g, 5.95×10⁻³ mol) and potassium iodide (0.07 g, 3.97×10⁻⁴ mol)were added dropwise and the reaction mixture then refluxed (48 hrs).Methanol (20 cm³) was then added to the reaction mixture followed bywater (75 cm³). The product was then extracted into ether (3×30 cm³) andthen the combined organic layers were washed with brine (2×20 cm³) anddried over magnesium sulphate. The solution was then filtered and thesolvent removed under reduced pressure and the crude product purified bycolumn chromatography on silica gel using a 3:7 ethylacetate/hexanemixture as eluent and recrystalised from cold propanone, producing thedesired white crystalline product.

[0065] Yield 0.44 g (36%) GC purity (99.9%). Cr—I=65° C. S_(B)-I=64° C.

EXAMPLE 2

[0066] Using analogous methods to that described in Example 1 but withdifferent starting materials, the following compounds were prepared.

trans, trans4′-(2-ethoxyethoxy)-4-pentylbicyclohexane (Compound 4 inTable 1)

[0067] Yield 0.43 g (33%) GC purity (99.8%). Cr—I=25° C.

trans, trans4′-(2-methoxyethoxy)-4-propylbicyclohexane (Compound 1inTable 1)

[0068] Yield 0.29 g (22%) GC purity (100%). Cr—I=47° C. N—I=44° C.

trans, trans-4′-(2-ethoxyethoxy)-4-propylbicyclohexane (Compound 3 inTable 1)

[0069] Yield 0.15 g (11%) GC purity (99.6%). Cr—I=20° C. N—I=7° C.

trans, trans-4′-(2-propoxyethoxy)-4-propylbicyclohexane (Compound 5 inTable 1)

[0070] Yield 0.08 g (12%) GC purity (100%). Cr—I=90° C.

EXAMPLE 3

[0071] Liquid Crystal Properties

[0072] The liquid crystal properties of the compounds of the inventionwere tested using conventional methods. In particular the melting andclearing points of compounds of the invention and structurally similarcompounds were determined for comparison purposes. The results are shownin Tables 2-6.

[0073] Table 2 shows for the trans, trans4-propyl,4′-R-bicyclohexyl corecompound the effect of the position and number of oxygen atomsincorporated in the 5 atom end chain. All members of the series exceptthe ethyleneoxy derivative exhibit a smectic B phase. Trans, trans-4-propyl,4′-pentylbicyclohexane shows only a smectic B phase. This isalso the behaviour when one of the carbons in the pentyl chain isreplaced with an oxygen. When, however, two carbons are replaced as inthe methyl ethyleneoxy derivative Compound No 1, the smectic B phase islost and a monotropic nematic phase is observed.

[0074] Table 3 shows for both the propyl and pentyl cyclohexyl analoguesthe effect of extending the ethyleneoxy chain by 1 carbon unit at theterminus. Extending the terminal chain by only 1 carbon atom has a verylarge effect on the melting point, which is substantially reduced in theC3 and C5 homologues Compounds 3 and 4 when compared with the shorterchain of Compounds 1 and 2. TABLE 2

Compound Iso No. R Cr SmB N Liq Comparative Example A

• 23 • 96 — • Comparative Example B

• 32 • 74 — • Comparative Example C

• 15 • 42 — • Comparative Example D

• 25 • 85 — • Compound no 1

• 47 (• 44) •

[0075] TABLE 3 Compound No. (Table 1) Cr SmB N Iso Liq 1 • 47 • (• 44) •3 • 20 • (• 7)  • 2 • 65 (•64) • 4 • 25 • 5 • 90 •

[0076] Table 4 shows the effect of the ethyleneoxy end chain with anumber of different cores. Replacement of the cyclohexyl group with aphenyl group in Comparative Example 1A leads to a reduction in thestability of the nematic phase, which remains a monotropic transition,as it does in the tolane. A nematic phase is not observed in thebiphenyl derivative. TABLE 4 Compound Iso No. Structure Cr N LiqCompound No 1

• 47 • (44) • Comparative Example 1A

• 41 • (13) • Comparative Example 1B

• 97 •

[0077] A similar trend in melting and clearing points was found in thepentyl derivatives.

[0078] Table 5 shows the same cores and end chain as Table 4 but herethe ethyleneoxy chain is extended by 1 carbon unit, to terminate in anethyl group instead of a methyl group. The overall trends are againdependant on the core, the melting points are however reduced. None ofthe compounds exhibit an enantiotropic nematic phase. TABLE 5 CompoundIso No. Structure Cr N Liq 3

• 20 • (7) • Comparative Example 3

• 13 • (<−20) •

[0079] Table 6, the ethyleneoxy end chain is sequentially increased by 1carbon unit from methyloxy- to propyloxy-ethenyloxy-bicyclohexyl. Themelting point falls and then rises substantially in a typical manner asthe end chain is increased in length. TABLE 6 Compound No. Iso (Table 1)Cr N Liq 1 • 47 • (44) • 3 • 20 • (7)  • 5 • 90 •

EXAMPLE 4

[0080] Physical Properties

[0081] The physical properties of a number of the compounds describedwere evaluated using conventional methods.

[0082] The dipole moments of the two compounds Comparative Example 2 andCompound No 2 were determined from extrapolation in solution in PCH32 at25° C. at 4, 6, 8 and 10% concentrations measured in a 9 μm cell. Thedipole moments for the two compounds appear very similar and are shownin Table 7. TABLE 7 Compound No. Structure μ(D) Comparative Example 2

2.32 Compound No 2

2.26

EXAMPLE 5

[0083] Properties of Mixtures

[0084] Mixtures of dicyclohexyl ethyleneoxy compounds in a standardbasic mixture ethyl linked phenyl cyclohexanes were made and the phasebehaviour, permittivities and birefringence determined. The mixtures,phase behaviour and structure of the compounds is listed in Table 8.None of the mixtures exhibited higher order smectic phases. TABLE 8Mixture Clearing number, Compound No point of % of and transitionmixture compound temperatures Structure (° C.) I 9.75% 2 K65(SmB 64)

49.9 II 9.94% 4 K27.2I

46.5

[0085] The permittivities of the mixtures are shown in Table 9. TABLE 9Mixture ε↑↑ ε⊥ Δε I 11.8684 5.1236 6.75 II 11.6578 5.1522 6.51 Mixtureethyl linked phenyl 12.95 5.13 7.82 cyclohexanes

[0086] The birefringence of the mixtures of 25° C. and 30° below theclearing point of the mixture are given in Table 10. All mixtures showlow birefringence. TABLE 10 Temperature Mixture (° C.) n_(e) n_(o) Δn I25 1.5713 1.4833 0.088 19.9 1.5755 1.4841 0.091 II 25 1.5683 1.48410.084 16.5 1.5750 1.4853 0.0897

1. The use of a compound of formula (I)

wherein R¹ is an alkyl or alkenyl group, either of which may beoptionally interposed with one or more oxygen or sulphur atoms, R² is analkyl or alkenyl group, Y′ and Y″ are independently selected from oxygenor sulphur, n is an integer from 1 to 5, X is a direct bond or aC₁₋₄alkylene or C₂₋₄alkenylene chain, and A is group comprising one ortwo rings which are independently selected from aryl, heterocylic orcycloalkyl rings, and when there are two rings, they are bonded togetherdirectly or by way of a C₂₋₄alkylene or C₂₋₄alkenylene group, forincreasing the u.v. stability of a liquid crystal mixture.
 2. The useaccording to claim 1 wherein the group A in formula (I) is selected froma group of formula (i), (ii), (iii), (iv) or (v)

where X′, X″ and X′″ are each selected from a direct bond, aC₁₋₄alkylene chain or a C₂₋₄alkenylene chain, and each group R_(a),R_(b), R_(c) and R_(d) are independently selected from CH₂ or oxygen. 3.The use according to claim 2 wherein R_(a), R_(b), R_(c) and R_(d) wherepresent, are CH₂ groups.
 4. The use according to any one of thepreceding claims wherein Y′ and Y″ in formula (I) are oxygen.
 5. The useaccording to any one of the preceding claims wherein n in formula (I) is2.
 6. The use according to any one of the preceding claims wherein R² informula (I) is a C₁₋₁₀alkyl.
 7. The use according to claim 6 wherein R²in formula (1) is C₁₋₃alkyl.
 8. The use compound according to any one ofthe preceding claims wherein X in formula (I) is a direct bond or aC₁₋₂alkylene chain.
 9. The use according to claim 8 wherein X in formula(I) is a direct bond.
 10. The use according to any one of claims 1 to 7wherein X is a C₂₋₄alkenylene chain of formula (x), (xi) or (xii)


11. The use according to any one of the preceding claims wherein thecompound of formula (1) is a compound of formula (IA)

where R¹, R², A, X, Y′, Y″ and n are as defined in claim
 1. 12. The useaccording to any one of the preceding claims wherein R¹ in formula (I)is a straight chain C₁₋₆alkyl group which optionally contains one or twooxygen or sulphur atoms.
 13. The use according to any one of claims 1 to11 wherein R¹ in formula (I) is a C₂₋₁₀alkenyl group.
 14. The useaccording to claim 13 wherein the alkenyl group R¹ contains two doublebonds.
 15. The use according to claim 14 wherein the alkenyl group R¹ isa diene of formula (VI) —(CH₂)_(m)—CH═CH—CH═CH—(CH₂)_(p)—CH₃   (VI)where m and p are 0 or an integer from 1-5, provided that m+p does notexceed
 5. 16. The use according to any one of the preceding claimswherein the compound of formula (I) is a compound of formula (III)

where R¹ and R² are as defined in claim
 1. 17. A method for preparing acompound of formula (I) as defined in anyone of the preceding claimswhich comprises reacting a compound of formula (IV)

where R¹, A, X and Y′ are as defined in relation to formula (I) in claim1; with a compound of formula (V) z-(CH₂)_(n)—Y″—R²   (V) where Y″ andR² is as defined in claim 1 and Z is a leaving group.
 18. A liquidcrystal device comprising an outdoor liquid crystal display device, or adevice comprising a phosphorescent substrate or a phosphor layer, saiddevice comprising a compound of formula (I) as defined in any one ofclaim 1 to
 16. 19. A liquid crystal device according to claim 18 whereinthe compound of formula (I) is in admixture with other liquid crystalcompounds.
 20. A liquid crystal device which comprises a display cellcomprising a layer of a liquid crystal material wherein the liquidcrystal material comprises a compound of formula (I) as defined in claim1, means for addressing the liquid crystal material so as to allow lightto pass through it when appropriately addressed, and an emitting layercomprising phosphor elements, arranged to receive light passing throughthe liquid crystal layer.
 21. A compound of formula (VII)

wherein R¹⁰ is an allyl or alkenyl either of which may be optionallyinterposed with one or more oxygen or sulphur atoms, R¹¹ is an alkyl oralkenyl group, Y′, Y″, n, X and A are as defined in claim 1, providedthat when A is cycloalkyl, X is a direct bond or a C₂alkylene chain, nis 2 an Y′ and Y″ are both oxygen, then when R₁₁ is C₁₋₄ n-alkyl, R₁₀ isan alkenyl group, a branched alkyl or an n-alkyl having more than 8carbon atoms or when R₁₀ is a C₁₋₇ n-alkyl group, R₁₁ is an alkenylgroup, a branched alkyl or an n-alkyl having more than 5 carbon atoms.22. A compound according to claim 21 of formula (VIIA)

wherein R¹²is an alkyl or alkenyl group, either of which may beoptionally interposed with one or more oxygen or sulphur atoms, R¹³ isan alkyl or alkenyl group, Y′, Y″, n, and X ate as defined in claim 1,and A is a group comprising one or two rings which are independentlyselected from aryl or hetocylic, and when there are two rings, they arebonded together directly or by way of a C₁₋₄alkylene or C₂₋₄alkenylenegroup.
 23. A compound according to claim 21 of formula (VIIB)

where Y′, Y″ and n are defined in claim 1, R¹⁴ is an alkyl or alkenylgroup, either of which may be optionally interposed with one or moreoxygen or sulphur atom, and R¹⁵ is an alkyl or alkenyl group, providedthat when X is a direct bond or a C₂alkylene chain, n is 2 and Y′ and Y″ate both oxygen, then when R₁₁ is C₁₋₄alkyl, R₁₀ is an alkenyl group, abranched alkyl or an n-alkyl having more than 8 carbon atoms or when R₁₀is a C₁₋₇ n-alkyl group, R₁₁ is an alkenyl group, a branched alkyl or ann-alkyl having more than 5 carbon atoms.